radeonfb: misc cleanup of engine and dst cache handling

Fix a couple of incomplete tests of the chip families in the engine
init/reset code and proper initialization of the destination cache mode.
The result should better match what the latest X radeon driver does.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: David S. Miller <davem@davemloft.net>
Cc: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>